The UN’s Rio+20 conference occurred in June, without consequences. A generation has passed since the 1992 Earth Summit in Rio, which begat other conferences and protocols. And, by now, apocalypse fatigue — boredom from being repeatedly told the end is nigh.

This began two generations ago, in 1972, when we were warned (by computer models developed at MIT) that we were doomed.

We were supposed to be pretty much extinct by now, or at least miserable. We are neither. So, what went wrong?

That year begat “The Limits to Growth,” a book from the Club of Rome, which called itself “a project on the predicament of mankind.” It sold 12 million copies, staggered The New York Times (“one of the most important documents of our age”) and argued that economic growth was doomed by intractable scarcities.

Bjorn Lomborg, the Danish academic and “skeptical environmentalist,” writing in Foreign Affairs, says it “helped send the world down a path of worrying obsessively about misguided remedies for minor problems while ignoring much greater concerns,” such as poverty, which only economic growth can ameliorate.

MIT’s models foresaw the collapse of civilization because of “nonrenewable resource depletion” and population growth.

“In an age more innocent of and reverential toward computers,” Lomborg writes, “the reams of cool printouts gave the book’s argument an air of scientific authority and inevitability” that “seemed to banish any possibility of disagreement.”

Then — as now, regarding climate change — respect for science was said to require reverential suspension of skepticism about scientific hypotheses. Time magazine’s story about “The Limits to Growth” exemplified the media’s frisson of hysteria:

“The furnaces of Pittsburgh are cold; the assembly lines of Detroit are still. In Los Angeles, a few gaunt survivors of a plague desperately till freeway center strips . . . Fantastic? No, only grim inevitability if society continues its present dedication to growth and ‘progress.’”

Technological innovations have replaced mercury in batteries, dental fillings and thermometers, mercury consumption is down 98 percent and its price was down 90 percent by 2000.

Since 1970, when gold reserves were estimated at 10,980 tons, 81,410 tons have been mined and estimated reserves are 51,000 tons. Aluminum consumption has increased 16-fold since 1950, the world has consumed four times the 1950 known reserves, and known reserves could sustain current consumption for 177 years. Potential US gas resources have doubled in the last six years. And so on.

The modelers missed something — human ingenuity in discovering, extracting and innovating. Which did not just appear after 1972.

Aluminum is one of earth’s most common metals. But until the 1886 invention of the Hall-Heroult process, it was so difficult and expensive to extract that “Napoleon III had bars of aluminum exhibited alongside the French crown jewels.”

Forty years after “The Limits to Growth” imparted momentum to environmentalism, that impulse now is often reduced to children indoctrinated to “reduce, reuse, and recycle.” Lomborg calls recycling “a feel-good gesture that provides little environmental benefit at a significant cost.”

In 1980, economist Julian Simon made a wager in the form of a complex futures contract. He bet Paul Ehrlich (whose 1968 book “The Population Bomb” predicted “hundreds of millions of people” would starve to death in the 1970s as population growth swamped agricultural production) that by 1990 the price of any five commodities Ehrlich and his advisers picked would be lower than in 1980. Ehrlich’s group picked five metals. All were cheaper in 1990.

The bet cost Ehrlich $576.07. But that year he was awarded a $345,000 MacArthur Foundation “genius” grant and half of the $240,000 Crafoord Prize for ecological virtue. One of Ehrlich’s advisers, John Holdren, is Barack Obama’s science adviser.